ZnS:Cu electroluminescent phosphor and method of making same

ABSTRACT

A improved ZnS:Cu electroluminescent phosphor having a halflife of at least about 900 hours. The halflife improvement is made by doping the phosphor with minor amounts of gold and substantially increasing the amount of low intensity milling between firing steps. The improved phosphor has a dramatically longer halflife without sacrificing brightness or exhibiting large shifts in emission color.

CROSS-REFERENCES TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 08/637032,filed concurrently herewith, which is incorporated herein by reference.

CROSS-REFERENCES TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 08/637032,filed concurrently herewith, which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to ZnS:Cu electroluminescent phosphors. Moreparticularly, it relates to a ZnS:Cu electroluminescent phosphor havingimproved halflife and brightness characteristics.

BACKGROUND ART

Electroluminescent (EL) phosphors are incorporated into thick film ACelectroluminescent devices used for backlighting liquid crystal displays(LCD), for automotive dashboard and control switch illumination, and foremergency egress lighting. Some important characteristics of these ELdevices include brightness, color and halflife. Such characteristicsdepend inherently on the phosphors which are incorporated into the ELdevices.

One particularly important class of EL phosphors are thecopper-activated zinc sulfide phosphors, ZnS:Cu, which have commerciallydesirable brightness and color characteristics. Such phosphors may beblue, blue-green, green or yellow-orange (co-activated with manganese)emitting. U.S. Pat. No. 4,859,361 to Reilly et al., which is herebyincorporated by reference, describes generally how to makecopper-activated zinc sulfide phosphors. First, the phosphor precursormaterials, ZnS, a copper source and a chloride flux are mixed togetherand heated to form a hexagonal ZnS material containing copper andchlorine ions. Next, the hexagonal ZnS is subjected to low intensitymilling to convert some of the hexagonal ZnS to its cubic crystallineform. And finally, the milled material is blended with zinc sulfate andcopper sulfate and refired at a lower temperature to form anelectroluminescent phosphor.

JP 4-270780 discloses improving the halflife of a copper-activated zincsulfide phosphor to about 410 hours by incorporating small amounts ofgold into the phosphor. U.S. Pat. No. 5,110,499 to Reilly furtherimproves the halflife of copper-activated zinc sulfide phosphors toabout 450 hours by increasing the average particle size of the phosphor.WO 91/16722 to Faria discloses that increasing the mechanical stressingof the hexagonal ZnS material between the first and second firing stepsincreases the halflife of the phosphor but at the expense of brightness.

Since many of the applications for EL phosphors are now requiringhalflife performances on the order of 1000 hours without sacrificingbrightness, it would be a significant advantage to produce acopper-activated zinc sulfide EL phosphor having a longer halflife whilemaintaining or improving the desirable brightness and colorcharacteristics.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to provide an electroluminescentphosphor having an improved halflife without sacrificing brightness oremission color.

It is a further object of the invention to provide a method forincreasing the halflife of a ZnS:Cu phosphor without substantiallyaffecting its brightness or emission color.

In accordance with one aspect the invention, there is provided anelectroluminescent phosphor having the general formula ZnS:Cu,Cl,Au, ahalflife of at least about 900 hours, and x and y color coordinateswhere the x color coordinate is from about 0.156 to about 0.196 and they color coordinate is from about 0.370 to about 0.430.

In accordance with another aspect the invention, there is provided amethod for increasing the halflife of a ZnS:Cu electroluminescentphosphor comprising:

blending amounts of zinc sulfide, a copper source, a gold source and achloride flux to form a mixture;

firing the mixture at a temperature from about 900° C. to about 1400° C.for about 2 hours to about 8 hours to form a zinc sulfide materialcontaining copper, chlorine and gold;

subjecting the zinc sulfide material to a low intensity milling forbetween about 190 minutes to about 280 minutes to form a milledmaterial;

blending the milled material with an amount of zinc sulfate and anamount of copper sulfate; and

refiring the milled material at a temperature from about 500° C. toabout 1000° C. for about 1 to 5 hours to form an electroluminescentphosphor.

In accordance with yet another aspect the invention, there is providedan electroluminescent lamp comprising:

a first electrode, a first layer of dielectric material adjacent to thefirst electrode, a second layer of a dielectric material adjacent to thefirst layer;

the second layer containing an electroluminescent phosphor having ageneral formula ZnS:Cu,Cl,Au, a halflife of at least about 900 hours,and x and y color coordinates where the x color coordinate is from about0.156 to about 0.196 and the y color coordinate is from about 0.370 toabout 0.430; and

a second electrode adjacent to the second layer of dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the structure of anelectroluminescent lamp.

PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

FIG. 1 is a schematic representation of the structure of anelectroluminescent lamp 60. A conductive substrate material, such asaluminum or graphite, forms a first electrode 62 of the lamp 60, while atransparent conductive film, such as indium tin oxide, forms a secondelectrode 64. Sandwiched between the two conductive electrodes 62 and 64are two additional layers of dielectric material 70 which can be, forexample, cyanoethyl cellulose or cyanoethyl starch. Adjacent to thefirst electrode 62 is a layer of dielectric material 70 in which may beembedded particles of a ferroelectric material 72 such as bariumtitanate. Adjacent to the second electrode 64 is a layer of dielectricmaterial 70 in which may be embedded particles of the electroluminescentphosphor 74 of this invention.

The halflife of an electroluminescent lamp as used herein is defined asthe time it takes for the brightness of the EL lamp to reach half itsinitial value. The initial brightness of the lamp is typically measuredafter a 24 hour burn-in period. The burn-in period is used to stabilizethe brightness of the lamp and increase the reproducibility of themeasurement. Brightness is typically measured in footlamberts (fL) atconditions of 100 volts and 400 hertz.

A typical copper-activated zinc sulfide phosphor is ZnS:Cu,Cl which isavailable from OSRAM SYLVANIA INC. of Towanda, PA. For example, OSRAMSYLVANIA Type 723 ZnS:Cu,Cl has CIE color coordinates where x is 0.176±0.02 and y is 0.400 ±0.03. The typical halflife of this phosphor isabout 400 hours. The halflife of this phosphor can be improved tobetween 600-800 hours by increasing copper content. However, this causesan undesirable color shift in the emission spectrum of phosphorresulting in a green emission color where the y color coordinate isincreased to about 0.480 to 0.490.

We have found that the halflife of ZnS:Cu phosphors can be significantlyimproved by doping the phosphors with minor amounts of gold andincreasing the amount of low intensity milling between the first andsecond firings. The resultant ZnS:Cu phosphor has a halflife of at least900 hours and exhibits an average increase in brightness while onlyexperiencing a slight shift in its average y color coordinate of lessthan or equal to about 0.03.

In a general method, the phosphor is made by combining zinc sulfide, acopper source, a gold source and a chloride flux. Other components suchas sulfur and ZnO may also be added. The mixture is then fired at atemperature from about 900° C. to about 1400° C. for about 2 to about 8hours to produce hexagonal ZnS containing copper, chlorine and gold.Preferred first step firing conditions are 1100° C. to 1300° C. for 4 to6.5 hours. Water washing is used to remove any residual chloride flux.

At this point, the ZnS is not yet substantially electroluminescent. Inorder to make it electroluminescent, the ZnS is subjected to a lowintensity milling to induce the transformation of the ZnS to its cubiccrystalline form. The amount of low intensity milling used in thismethod is substantially greater than the amount indicated by the priorart. For example, U.S. Pat. No. 4,859,361 discloses milling for 90minutes between firings and WO 91/16722 teaches mechanically stressingthe phosphor precursor for up to about 120 minutes. In the instantinvention, the preferred amount of low intensity milling is at least 50%longer than the prior art, between about 190 to about 280 minutes.

After milling, the ZnS is blended with amounts of copper sulfate andzinc sulfate and fired at a temperature from about 500° C. to about1000° C. for about 1 to about 5 hours to form the electroluminescentphosphor. Preferred second step firing conditions are 650° C. to 850° C.for 1.5 to 3 hours. After cooling, the phosphor is given additionalwashes including washing with acetic acid to remove copper oxide andunreacted ZnO and washing with a KCN solution to remove superficialcopper. Final finishing steps include water washing, filtering, dryingand sieving to -325 mesh.

The following non-limiting examples are presented to enable thoseskilled in the art to more clearly understand and practice the presentinvention. These examples should not be considered as a limitation uponthe scope of the present invention, but merely as being illustrative andrepresentative thereof.

Luminescent grade S-10 ZnS from OSRAM SYLVANIA containing 1 wt. %chloride was doped with 0.5 wt. % gold using a solution of goldchloride. The S-10 ZnS is slurried with deionized (DI) water and thegold solution is added. The gold doped slurry is then dried. The golddoped S-10 was mixed in varying proportions with amounts of S-10 ZnScontaining 1 wt. % chloride to produce ZnS mixtures containing differentamounts of gold. The relative proportions of the two S-10 blends aregiven in Table 1. To each of these mixtures, 23.87 g CuSO₄, 1270.06 g S,79.45 g ZnO, 476.27 g BaCl₂, 476.27 MgCl₂ and 317.51 g NaCl were added.The latter three components, BaCl₂, MgCl₂ and NaCl, are flux materials.

The mixtures were fired in covered crucibles at 1205° C. for 5 1/4hours. The fired cakes were removed from the furnace and washed toremove the flux material. Approximately three hot DI water washes (0.7gal./lb) were needed to remove the flux. (The chloride level in thesupernatant liquid can be tested by either measuring the conductivity ofthe liquid or precipitating AgCl with AgNO₃.) The washed material wassubjected to slight milling using a Simpson Mix-Muller in 35 lb.quantities for different time periods to induce the transformation ofhexagonal ZnS to cubic ZnS. The milled material was blended withZnSO₄.7H₂ O and CuSO₄ in the ratio of 1159 g of ZnSO₄.7H₂ O and 113.5 gof CuSO₄ per every 10 lbs. of milled material and fired in an electricfurnace for about 2 1/4 hours at 730° C. After firing, the fired cakeswere removed from the furnace, cooled to room temperature, blended andwashed with DI water. After two or three DI water washes, the materialwas washed with acetic acid (1 gal. glacial acetic acid/5 gal. DI water)to remove copper oxide and unreacted ZnO. After subsequently washing atleast twice with DI water, the material was washed with KCN (2 lbs.KCN/5 gal. DI water) to remove most of the superficial copper from thephosphor. The KCN wash changes the body color of the material from adark gray to almost colorless. The phosphor was then washed with DIwater, filtered, dried and sieved to -325 mesh. The phosphors wereincorporated into conventional thick film EL devices and evaluated forbrightness, halflife and color. The results of which are given in Table2. A Type 723 control sample made without using the gold dopant or thelonger milling period is provided for comparison.

                  TABLE 1                                                         ______________________________________                                                               S-10 with                                                     Au-doped S-10 with                                                                            1 wt. % Cl                                                                             Milling Time                                  No.    1 wt. % Cl (g)  (g)      (min)                                         ______________________________________                                        1      142.88          15732.85 200                                           2      95.25           15780.48 225                                           3      142.88          15732.85 250                                           4      47.63           15828.10 200                                           5      95.25           15780.48 225                                           6      95.25           15780.48 260                                           7      162.38          15713.35 225                                           8      95.25           15780.48 225                                           9      95.25           15780.48 225                                           10     47.63           15828.10 250                                           11     95.25           15780.48 190                                           12     95.25           15780.48 225                                           13     28.12           15847.61 225                                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                      Milling Initial                                                 Sample                                                                              wt. %   Time    Brightness                                                                           x color                                                                              y color                                                                              Halflife                           No.   gold    (min)   (fL)   coordinate                                                                           coordinate                                                                           (h)                                ______________________________________                                        Control                                                                             0.00     85     26.5   0.176  0.392   437                               1     0.0045  200     26.6   0.186  0.422  1051                               2     0.003   225     28.8   0.186  0.423  1020                               3     0.0045  250     27.9   0.184  0.416   943                               4     0.0015  200     28.9   0.182  0.408   967                               5     0.003   225     29.4   0.182  0.411   923                               6     0.003   260     30.2   0.184  0.423  1134                               7     0.00512 225     28.9   0.187  0.434  1254                               8     0.003   225     29.4   0.186  0.426  1159                               9     0.003   225     30.1   0.186  0.431  1160                               10    0.0015  250     27.3   0.185  0.422  1421                               11    0.003   190     25.1   0.185  0.425  1634                               12    0.003   225     29.2   0.183  0.419  1122                               13    0.0088  225     28.5   0.184  0.426  1183                               ______________________________________                                    

The results given in Table 2 show a dramatic increase in the halflife ofthe EL phosphor and an average increase in its brightness as compared tothe control sample. The average halflife of the Type 723 phosphors madeby this method was 1100 ±200 hours which is about 2.5 times the halflifeof the control sample and the average brightness value was 28 ±1 fLwhich represents an average increase of about 6% over the control. Withregard to emission color, the average x color coordinate value for thephosphors made by this method was 0.185 ±0.002 which is an increase ofonly about 0.009 over the x value for the control and the average ycolor coordinate value for these phosphors was 0.422 ±0.007 which is anincrease of about 0.030 over the y value for the control. Both colorcoordinates are still within the acceptable limits for Type 723phosphors. Thus, the halflife of this phosphor has been dramaticallyincreased while maintaining emission color and increasing the averagebrightness.

Chemical analysis of the phosphor samples found that for all samples thechlorine concentration was about 0.03 wt. % and the copper concentrationwas about 0.06 wt. %. Unexpectedly, the amount of gold detected in thephosphors was considerably less than the amount added to the initialmixture before firing. The amount of gold detected in the phosphorsranged from 0.27 to 0.87 ppm whereas the amount of gold added initiallyranged from 15 to 88 ppm. The average gold concentration in the sampleswas about 0.5 ±0.2 ppm.

Another typical ZnS:Cu,Cl phosphor is green emitting OSRAM SYLVANIA Type728 phosphor. Type 728 has a higher copper content than Type 723 whichcauses it to have a greener emission color. The typical colorcoordinates for this phosphor are x=0.182 ±0.20 and y=0.455 ±0.30 andthe average halflife is 550 hours. Two samples of this phosphor typewere prepared according to the above method except that for each sample158.54 g gold doped S-10 ZnS (1 wt. % Cl), 15717.20 g S-10 ZnS (0.1 wt.% Cl), and 33.88 g CuSO₄ were used in the initial mixtures. The amountof low intensity milling used between firings was 280 minutes. Bothsamples were incorporated into lamps and their emission characteristicswere measured. The results of the measurements are given Table 3.Average values for a Type 728 control phosphor are provided forcomparison.

                  TABLE 3                                                         ______________________________________                                                      Milling Initial                                                 Sample                                                                              wt. %   Time    Brightness                                                                           x color                                                                              y color                                                                              Halflife                           No.   gold    (min)   (fL)   coordinate                                                                           coordinate                                                                           (h)                                ______________________________________                                        Con-  0.00     85     27.5   0.183  0.447   547                               trol*                                                                         14    0.005   280     29.6   0.194  0.460  1425                               15    0.005   280     30.1   0.197  0.482  1212                               ______________________________________                                         *Average values                                                          

For the first Type 728 sample, No. 14, the halflife is at least about1200 hours and, for the second sample, No. 15, the halflife is at leastabout 1400 hours. Both Type 728 samples exhibit a halflife more thantwice the average value for the control sample. There has also been anincrease in brightness of at least about 8% over the control. Moreover,for each sample, the x and y color coordinates are within the specifiedrange for Type 728 phosphors, i.e., an x value from about 0.162 to about0.202 and a y value from about 0.425 to about 0.485.

The incorporation of minor amounts of gold and the substantiallyincreased low intensity milling between the first and second firing isexpected to produce similar benefits for other types of ZnS:Cu ELphosphors. In particular, the halflife of yellow-orange emittingZnS:Cu,Mn phosphors should be increased without exhibiting unacceptablechanges in their emission color or brightness.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. An electroluminescent phosphor having the general formulaZnS:Cu,Cl,Au, a halflife of at least about 900 hours, and x and y colorcoordinates where the x color coordinate is from about 0.156 to about0.196 and the y color coordinate is from about 0.370 to about 0.430. 2.The electroluminescent phosphor of claim 1 wherein the phosphor has ahalflife of at least about 1100 hours.
 3. The electroluminescentphosphor of claim 1 wherein the phosphor has a halflife of at leastabout 1300 hours.
 4. The electroluminescent phosphor of claim 1 whereinthe phosphor has an initial brightness of at least about 28 fL.
 5. Theelectroluminescent phosphor of claim 1 wherein the phosphor has a ycolor coordinate of about 0.422.
 6. The electroluminescent phosphor ofclaim 1 wherein the amount of gold in the phosphor is about 0.5 ±0.2ppm.
 7. The electroluminescent phosphor of claim 1 wherein the phosphorhas a copper concentration of about 0.06 wt. % and a chlorineconcentration of about 0.03 wt. %.
 8. The electroluminescent phosphor ofclaim 1 wherein the x color coordinate is from about 0.162 to about0.202 and the y color coordinate is from about 0.425 to about 0.485. 9.The electroluminescent phosphor of claim 8 wherein the phosphor has ahalflife of at least about 1200 hours.
 10. The electroluminescentphosphor of claim 8 wherein the phosphor has a halflife of at leastabout 1400 hours.
 11. A method for increasing the halflife of a ZnS:Cuelectroluminescent phosphor comprising:blending amounts of zinc sulfide,a copper source, a gold source and a chloride flux to form a mixture;firing the mixture at a temperature from about 900° C. to about 1400° C.for about 2 hours to about 8 hours to form a zinc sulfide materialcontaining copper, chlorine and gold; subjecting the zinc sulfidematerial to a low intensity milling for between about 190 minutes toabout 280 minutes to form a milled material; blending the milledmaterial with an amount of zinc sulfate and an amount of copper sulfate;and refiring the blended milled material at a temperature from about500° C. to about 1000° C. for about 1 to 5 hours to form anelectroluminescent phosphor.
 12. The method of claim 11 wherein theamount of gold in the mixture is from about 15 to 88 ppm.
 13. The methodof claim 11 wherein the phosphor contains copper and chlorine.
 14. Themethod of claim 11 wherein an amount of a manganese source is added tothe mixture in order to produce a manganese and copper coactivatedphosphor.
 15. An electroluminescent lamp comprising:a first electrode, afirst layer of dielectric material adjacent to the first electrode, asecond layer of a dielectric material adjacent to the first layer; thesecond layer containing an electroluminescent phosphor having a generalformula ZnS:Cu,Cl,Au, a halflife of at least about 900 hours, and x andy color coordinates where the x color coordinate is from about 0.156 toabout 0.196 and the y color coordinate is from about 0.370 to about0.430; and a second electrode adjacent to the second layer of dielectricmaterial.
 16. The electroluminescent lamp of claim 15 wherein thephosphor has a halflife of at least about 1100 hours.
 17. Theelectroluminescent lamp of claim 15 wherein the phosphor has a halflifeof at least about 1300 hours.
 18. The electroluminescent lamp of claim15 wherein the phosphor has an initial brightness of at least about 28fL.
 19. The electroluminescent lamp of claim 15 wherein the phosphor hasa y color coordinate of about 0.422.
 20. The electroluminescent lamp ofclaim 15 wherein the amount of gold in the phosphor is about 0.5 ±0.2ppm.
 21. The electroluminescent lamp of claim 15 wherein the phosphorhas a copper concentration of about 0.06 wt. % and a chlorineconcentration of about 0.03 wt. %.
 22. The electroluminescent lamp ofclaim 15 wherein the x color coordinate is from about 0.162 to about0.202 and the y color coordinate is from about 0.425 to about 0.485. 23.The electroluminescent lamp of claim 22 wherein the phosphor has ahalflife of at least about 1200 hours.
 24. The electroluminescent lampof claim 22 wherein the phosphor has a halflife of at least about 1400hours.